Constant velocity joints (CVJs), also known as homokinetic joints, are used in joints between two rotating shafts to transmit rotation from one shaft to another at a constant velocity even if the angle between the shafts varies. This type of joint is commonly used in automobiles to enable transmission of torque from the engine to the wheels. Some constant velocity joints also allow for sliding contact between the shafts, for example, to allow the distance between the differential and the wheel (where this type of joint is widely used) to shorten and lengthen with the strokes of the suspension.
Some CVJs are ball and groove type joints having a cup-shaped outer housing, an inner race, a cage and a plurality of balls. The outer housing and the inner race are machined with grooves. The cage, placed between them, is machined with windows. The balls are placed in the windows so as to be able to move in the grooves of the outer housing and the inner race. This allows the outer housing and the inner race to move relative to each other, and thereby change the angle between their respective rotation axes while continuing to transmit torque from one to the other. The ball bearings held between the outer housing and the inner race rotate in the respective grooves so that a constant rotational speed can be maintained between the outer housing and the inner race even when their rotation axes are not aligned. Examples of such ball and groove type CVJs include fixed ball-type joints, also known as Rzeppa joints, and plunging ball-type joints. Fixed ball-type joints only permit rotation of the outer housing and inner race with respect to each other while plunging ball-type joints also allow axial motion between the outer housing and inner race.
In all of these ball and groove type CVJs, friction between the ball bearings and the groove surfaces generates heat in the joint which must be dissipated. The amount of heat generated depends on the rotational speed of the shafts, the torque exerted on the joint, and the operating angle of the joint. Heat generated at low operating angles can be dissipated through the outer housing to maintain reasonable operating temperatures in the joint. At large operating angles, however, the amount of heat generated can be too much to be dissipated effectively via the housing. The resulting high temperatures can damage the lubrication as well as the rubber boot protecting the CVJ from dust and dirt. Excessive heat can also negatively affect the metallic parts in the CVJ.
To aid heat dissipation, the external surface of the outer housing is sometimes provided with fins or grooves. Since all the torque in the driveline is transmitted through the outer housing, the outer housing must be sufficiently resistant to withstand these torques. The outer housing is thus typically made of steel, and its walls are made thicker in order to create the fins and grooves. While the fins and grooves can aid in dissipating some of the excess heat, they also significantly increase the weight of the CVJ.
There is thus a need for a CVJ that can operate at large operating angles, with high torques, and at high rotational speeds while effectively dissipating heat generated under these operating conditions.